An important design rule for a hearing aid is the requirement of a large signal to noise ratio, which is mainly determined by that of the microphone and its preamplifier. It will be shown that in order to increase the signal to noise ratio it is favourable to integrate the preamplifier with the microphone, which implies that the microphone should be made of silicon, preferably with a single wafer technology. For the development of a tonometer for the measurement of intraocular pressure, the application of a silicon force sensor rationalizes that also the flattening of the eye globe is measured with a silicon applanation sensor, instead of by optical means which is the present practice. A sensor construction has been developed, which combines a force, pressure and applanation sensor, all made in silicon.